This has got me puzzled.
If i run LXterminal and then su to gedit some /etc file, it fails as per this example:
[bb@townsend ~]$ su
[root@townsend bb]# gedit /etc/hosts
GLib-GIO:ERROR:gdbusconnection.c:2279:initable_init: assertion failed: (connection->initialization_error == NULL)
If I try it with geany it works OK.
Similarly, if I run the root terminal from the menu (i.e. the xvt script) then gedit works.